1. Field of the Invention
The present invention relates to solid state relays and more particularly to a solid state relay with very low power consumption.
2. Description of the Prior Art
For years, electromechanical relays have been used in a wide variety of power control electrical applications. Though these mechanical devices, which are built of a coil and contacts, have demonstrated considerable reliability, they suffer from the problems associated with having moving parts. A mechanical relay is subject to arcing and sparking, and in applications where it is required to switch a high DC voltage, the cost of a mechanical relay grows very rapidly. The coil switching leads to voltage spikes or fly-back voltage. In high power relays, as much as ten watts may be required to control the coil. Material fatigue can shorten the life of mechanical relays, and reliability can suffer due to shock and vibration.
These types of mechanical issues can be of major concern when relays are used in harsh environments. For example, many vehicles such as cars, tractor/trailers, heavy equipment and aircraft have a wide variety of relays in their various systems. These relays are subject to constant vibration from operation of the vehicle. Furthermore, many of the relays are built with contacts exposed to environmental conditions that might prove corrosive and lead to their failure (liquids, gases and other forms).
Since semiconductor devices have become widely available, and their cost has decreased, they have started to replace electromechanical relays. Previously in applications where a high power relay was required, normally closed terminals were required, and this substitution was unavailable. While it is possible to build a solid-state relay with normally closed terminals with comparable ratings (high current and low voltage) using a large number of depletion mode MOSFETs (D-MOSFET), the extremely high cost and large size of such a device makes it generally impractical. In the case of a high voltage, high current normally closed solid-state relay (SSR-NC), not D-MOSFET device existed in the prior art. It would be advantageous to have a solid-state relay that could be normally closed that solved these problems.
Solid-state and mechanical relays typically require substantial input power (control signal) for controlling and maintaining ON or OFF states of the output terminals. This power consumption is magnified in the case of an electromechanical relay. In come cases 10-20 watts is needed to move the terminals with the required speed. This makes regular (non-latchable) relays unusable in portable, low power applications.
An electromechanical lockable relay partially overcomes the problem with high power consumption since it is controlled with a short pulse, but it is bulkier, expensive to make, and more important, it has an inherent weakness—that of a flip-flop. It settles randomly at power-up into either of the two available states. Special extra circuitry must be used to insure it always settles into the desired initial state. It would also be advantageous to have a solid-state relay that solved these problems.